1.EL expression

1.1.EL concept

EL：Expression Language，表达式语言
作用: Replace and simplify JSP pages written in Java code

1.2.EL grammar

${表达式}
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!!! Note: JSP默认支持EL, so if you want to output "3> 4" string style how to do it? There are two ways:

使用isELIgnoreBut this approach would be that all the pages of EL expressions are ignored
使用“\”忽略个别

1.3.EL use

1.3.1. Operation

Operator:

Arithmetic operators: +, -, *, / (div),% (mod)
Comparison operators:>, <, =,> =, <=, ==,! =
Logical operators: && (and), || (or), | (not)
Air operators: empty

$ {Empty list}: for判断字符串、集合、数组对象是否为null或者长度是否为0

$ {Not empty str}: Analyzing string represented, set array object is not null and the length is greater than 0

1.3.2. Getting value

!!! Note: EL表达式只能从域对象中获取值
Syntax:

1. $ {domain name} key city:从指定域中获取指定的键值

Domain Name City:

pageContext：pageScope

request：requestScope

session：sessionScope

application(ServletContext)：applicationScope

2 $ {name} key: indicates a从最小的域中去查找是否有该键对应的值，直到找到为止
3. Get 对象、List集合、Map集合value

Target: $ {. The domain name of the city of key attribute name}

go getter method calls the object's essence

List collection: $ {domain key city name [index]}

Map set: $ {name} .key domain key name or $ {name domain key [ "key name"]}

java.class

package xpu.edu.web.servlet.EL;

import java.util.Date;

public class User {
    private String name;
    private int age;
    private Date birthday;

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
}

JSP page (Markdown seemingly does not support JSP code highlighting, all a bit ugly, will look)

<%@ page import="xpu.edu.web.servlet.EL.User" %>
<%@ page import="java.util.*" %><%--
 Created by IntelliJ IDEA.
 User: 15291
 Date: 2019/6/4
 Time: 21:27
 To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
   <title>获取数据</title>
</head>
<body>

<%
   User user = new User();
   user.setAge(18);
   user.setName("邹鸡儿");
   user.setBirthday(new Date());

   request.setAttribute("u",user);

   List list = new ArrayList();
   list.add("邹鸡儿");
   list.add("邹大炮");

   request.setAttribute("list",list);

   Map hashMap = new HashMap();
   hashMap.put("name","邹鸡儿");
   hashMap.put("age","19");

   request.setAttribute("hashMap",hashMap);
%>

<h3>获取对象中的值</h3>
${requestScope.u}<br/>

<%--
   通过对象的属性来获取
   * setter或getter方法，去掉set或get，在剩余部分首字母变小写
   * setName——>Name——>name
--%>

${requestScope.u.name}<br/>
${requestScope.u.age}<br/>
${requestScope.u.birthday}<br/>

<h3>获取List的值</h3>
${requestScope.list}<br/>
${requestScope.list[0]}
${requestScope.list[1]}
<%--
   越界时会输出空字符串""
--%>
${requestScope.list[10]}

<h3>获取Map的值</h3>
${requestScope.hashMap.name}<br/>
${requestScope.hashMap["name"]}

</body>
</html>

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1.4.EL implicit target

EL表达式有11个隐式对象, Focus on understanding pageContext: 可以获取JSP其他八个内置对象
$ {} pageContext.request.contextPath: dynamically acquire virtual directory
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2.JSTL

2.1.JSTL concept

JSTL: JavaServer Pages Tag Library, JSP Standard Tag Library is a free open source JSP tags provided by the Apache organization
effect:用于简化和替换JSP页面的Java代码

2.2.JSTL use the steps

Related jar package 1. Import
2. Introduction tag library: taglib directive: <% taglib%>
3. Use tag

2.3. Commonly used labels JSTL

if: The equivalent Java code if statement

Properties: test as an essential attribute, receiving boolean expression, if the expression is true, the tag is displayed if the content thereof, if false, the content is not displayed tag body

note:一般test属性会结合EL表达式一起使用

choose: The equivalent Java code switch statement

Choose to use the label statement: the equivalent switch statement

Label use when making a judgment: the equivalent case

Otherwise use the label to do otherwise declare: the equivalent of d

foreach: The equivalent Java code for statement

1. Complete Repeat

Properties:
1.begin: start value
2.end: end value
3.var: temporary variables
4.step: step
5.varStatus: circulation state of the object

index: the index container element, from zero
count: number of cycles, starting with 1

2. traverse the container

Properties:
1.item: container objects
2.var: temporary variable elements in the container
3.varStatus: circulation state of the object

index: the index container element, from zero
count: number of cycles, starting with 1

JavaWeb - EL and JSTL